ICs or LSIs, in a conventional way of producing an electronic circuit board, have been bonded onto a printed circuit board through soldering of the electrodes thereof to the printed circuit board. Soldering, however, does not enable effective bonding of the ICs or LSIs onto the printed circuit board, and does not contribute to an increase in the packaging density of ICs or LSIs.
In order to solve this, the BGA (ball grid array) technology has been developed which bonds ICs or LSIs onto the substrate using solder balls produced by forming solder into spherical shapes. According to this technology, it was possible to produce electric circuits while reconciling high productivity and high connection reliability by connecting the substrate, the chip and solder balls mounted on the substrate or the chip while melting solder balls at high temperature.
However, multi-layer substrates have been more used in recent years, and those multi-layer substrates, sensitive to the environmental conditions, are sometimes distorted, expanded or contracted to cause disconnection of the connection portions.
Patent Document 1 discloses, as a way of solving such a problem, conductive fine particles produced by forming a metal layer (conductive layer) containing highly conductive metal therein on the surface of each resin fine particle, and forming a low-melting point metal layer (solder layer) containing metal such as tin on the above metal layer. The conductive fine particles can ease the stress added thereto with the flexible resin fine particles, and enable, with the low-melting point metal layer formed on the outermost surface of each particle thereof, easy conductive connection between the electrodes.
The low-melting point metal layer, if formed on the surface of the metal layer through a method such as electroplating, has low-melting point metal deposited in the form of crystals and thus has a structure with fine irregularities on the surface thereof. In the case that the low-melting point metal layer has such a shape, the conductive fine particles are rubbed against each other during storage, which rubs the low-melting point metal at some parts off the particles. The rubbed-off metal pieces are oxidized to change into a black fine powder. Thereafter, the powder adheres to the surfaces of the conductive fine particles again through heteroaggregation. This phenomenon is what is called a blackening phenomenon. At the time of melting the conductive fine particles to mount, the blackening phenomenon inhibits the meltability of the low-melting point metal, resulting in poor mounting.
Patent Document 1 also discloses conductive fine particles each uniformly coated with a lubricant, as a way of solving the problem caused by the blackening phenomenon. Since such conductive fine particles are coated with the lubricant on the surfaces thereof, the particles are slightly bonded to each other when used, and are thus easily aggregated.
Patent Document 2 and Patent Document 3 each disclose a method of solving such a problem, i.e., a method of using conductive fine particles which have a molecular film of a metal soap or an organic film on the surface thereof. Those methods unfortunately bring a new problem that impurities are generated from the molecular film of a metal soap or the organic film, and thus the meltability of the low-melting point metal is inhibited.    Patent Document 1: Japanese Kokai Publication 2000-288771 (JP-A 2000-288771)    Patent Document 2: Japanese Kokai Publication 2006-9112 (JP-A 2006-9112)    Patent Document 3: Japanese Kokai Publication 2005-254246 (JP-A 2005-254246)